Intelligence and Miscellaneous Articles. 79 



Coins and cameos are reproduced very distinctly, even at the di- 

 stance of a millimetre, and notwithstanding the interposition of a 

 thin continuous sheet of mica, silver, or copper, provided only the 

 relief he strong and the temperature sufficiently elevated. 



If a paper on which a drawing had been traced in lamp-black, or 

 even wood-charcoal, be raised to a temperature sufficient to scorch 

 the paper, the portions of the reverse corresponding to the lines of 

 the drawing will be observed to have become more highly carbonized 

 than the rest. A similar eiFect may be observed with respect to the 

 lights and shades of variegated feathers or coloured woollen fabrics ; 

 that Is to say, the action of the heat is greater on the shades than 

 on the lights. If, while a coloured fabric Is being heated, it be kept 

 in contact -with paper impregnated with cyanide of potassium, the 

 shades produce a stronger impression than the lights. 



Tissues of different materials, shaded with black and white, or 

 with colours of various kinds, produce impressions on paper rendered 

 sensitive with the salts of gold and silver ; but the image Is very 

 uncertain : in general the dark tints are most decidedly marked ; 

 but In certain cases the light tints produce the strongest Impres- 

 sions, — these differences dei^endlng, no doubt, on the nature of the 

 colours used and of the mordants employed to fix them. Indeed, 

 colours produced by the same substance (as madder, for instance), 

 but fixed with different mordants, produce very unequal and very 

 various impressions. 



When cotton cloth Is dyed with Indigo, the pattern being left in 

 white, the blue background alone produced an impression ; whereas, 

 if the cotton be similarly dyed with prussIan blue, it Is the white 

 pattern which is reproduced. If either cotton cloth or porcelain be 

 covered with alternate stripes of indigo and prussian blue, the indigo 

 stripes produce an Image, the prussian blue do not. 



I have endeavoured to obtain images In the focus of a lens, which, 

 of course, ought to give a representation of the heated body, but 

 hitherto in vain. I do not know whether the images In the focus of 

 a concave mirror would prove more effective. 



The action which gives rise to the thermographic image is, without 

 doubt, very complex. The calorific radiation probably plays a con- 

 siderable part in producing the result ; but the material vapours 

 emanating from the heated body may also have some effect. In the 

 case of metallic bodies and dry wood, the action of the heat Is cer- 

 tainly preponderant ; and it seems to me to be clearly demonstrated 

 that, under certain circumstances, a sufficiently high temperature 

 produces effects analogous to those which we see every day resulting 

 from the action of light — the reduction, namely, of the salts of gold 

 and silver, tlic alteration of tissues, &c. 



In conclusion, I may be permitted to state that the experiments 

 described In this notice date from the month of January last, since 

 which time I have occasionally shown thermograph pictures to 

 several members of the Academy; and on the ^Dth of January I 

 performed. In the presence of Mr. Wheatstone, those exi^eriments 

 of which the 'Cosmos' makes mention in its number for February the 

 Uth.—Comptes Rendus, May 23, 1859. 



